Re: [PATCH -v4 2/8] locking/mutex: Rework mutex::owner

From	Will Deacon <will.deacon@arm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH -v4 2/8] locking/mutex: Rework mutex::owner
Date	2016-10-13 17:20 +0200
Message-ID	<srPXA-5cV-3@gated-at.bofh.it> (permalink)
References	<spEWB-2dt-5@gated-at.bofh.it> <spFfY-2lv-63@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

On Fri, Oct 07, 2016 at 04:52:45PM +0200, Peter Zijlstra wrote:
> The current mutex implementation has an atomic lock word and a
> non-atomic owner field.
> 
> This disparity leads to a number of issues with the current mutex code
> as it means that we can have a locked mutex without an explicit owner
> (because the owner field has not been set, or already cleared).
> 
> This leads to a number of weird corner cases, esp. between the
> optimistic spinning and debug code. Where the optimistic spinning
> code needs the owner field updated inside the lock region, the debug
> code is more relaxed because the whole lock is serialized by the
> wait_lock.
> 
> Also, the spinning code itself has a few corner cases where we need to
> deal with a held lock without an owner field.
> 
> Furthermore, it becomes even more of a problem when trying to fix
> starvation cases in the current code. We end up stacking special case
> on special case.
> 
> To solve this rework the basic mutex implementation to be a single
> atomic word that contains the owner and uses the low bits for extra
> state.
> 
> This matches how PI futexes and rt_mutex already work. By having the
> owner an integral part of the lock state a lot of the problems
> dissapear and we get a better option to deal with starvation cases,
> direct owner handoff.
> 
> Changing the basic mutex does however invalidate all the arch specific
> mutex code; this patch leaves that unused in-place, a later patch will
> remove that.
> 
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/mutex-debug.h  |   24 --
>  include/linux/mutex.h        |   46 +++--
>  kernel/locking/mutex-debug.c |   13 -
>  kernel/locking/mutex-debug.h |   10 -
>  kernel/locking/mutex.c       |  371 ++++++++++++++++++-------------------------
>  kernel/locking/mutex.h       |   26 ---
>  kernel/sched/core.c          |    2 
>  7 files changed, 187 insertions(+), 305 deletions(-)

Looks good to me:

Reviewed-by: Will Deacon <will.deacon@arm.com>

Will

Thread

[PATCH -v4 0/8] locking/mutex: Rewrite basic mutex Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:10 +0200
  [PATCH -v4 4/8] locking/mutex: Allow MUTEX_SPIN_ON_OWNER when DEBUG_MUTEXES Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:10 +0200
  [PATCH -v4 7/8] locking/mutex: Simplify some ww_mutex code in __mutex_lock_common() Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:10 +0200
  [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:30 +0200
    Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Will Deacon <will.deacon@arm.com> - 2016-10-13 17:20 +0200
      Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-17 12:50 +0200
        Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-17 15:30 +0200
          Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Boqun Feng <boqun.feng@gmail.com> - 2016-10-17 15:50 +0200
            Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-17 18:00 +0200
      Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-19 19:40 +0200
        ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex:  Restructure wait loop) Davidlohr Bueso <dave@stgolabs.net> - 2016-10-24 04:00 +0200
          Re: ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex:  Restructure wait loop) Kent Overstreet <kent.overstreet@gmail.com> - 2016-10-24 15:30 +0200
          Re: ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex:  Restructure wait loop) Kent Overstreet <kent.overstreet@gmail.com> - 2016-10-24 16:30 +0200
            Re: ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex:  Restructure wait loop) Eric Wheeler <bcache@lists.ewheeler.net> - 2016-10-25 19:00 +0200
              Re: ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex:  Restructure wait loop) Kent Overstreet <kent.overstreet@gmail.com> - 2016-10-25 19:50 +0200
    Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-18 15:20 +0200
  [PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of woken waiter Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:30 +0200
    Re: [PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of  woken waiter Will Deacon <will.deacon@arm.com> - 2016-10-13 17:30 +0200
      Re: [PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of  woken waiter Peter Zijlstra <peterz@infradead.org> - 2016-10-17 11:40 +0200
    Re: [PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of  woken waiter Peter Zijlstra <peterz@infradead.org> - 2016-10-18 14:30 +0200
  [PATCH -v4 2/8] locking/mutex: Rework mutex::owner Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:30 +0200
    Re: [PATCH -v4 2/8] locking/mutex: Rework mutex::owner Davidlohr Bueso <dave@stgolabs.net> - 2016-10-12 20:40 +0200
      Re: [PATCH -v4 2/8] locking/mutex: Rework mutex::owner Jason Low <jason.low2@hpe.com> - 2016-10-12 22:00 +0200
    Re: [PATCH -v4 2/8] locking/mutex: Rework mutex::owner Will Deacon <will.deacon@arm.com> - 2016-10-13 17:20 +0200
  [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid starvation Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:40 +0200
    Re: [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid  starvation Will Deacon <will.deacon@arm.com> - 2016-10-13 17:20 +0200
      Re: [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid  starvation Peter Zijlstra <peterz@infradead.org> - 2016-10-17 11:30 +0200
    Re: [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid  starvation Peter Zijlstra <peterz@infradead.org> - 2016-10-18 14:40 +0200
    Re: [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid  starvation Peter Zijlstra <peterz@infradead.org> - 2016-10-18 15:10 +0200
  [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:40 +0200
    Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:50 +0200
      Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Linus Torvalds <torvalds@linux-foundation.org> - 2016-10-07 18:00 +0200
        Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-07 18:20 +0200
      Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Thomas Gleixner <tglx@linutronix.de> - 2016-10-08 14:10 +0200
        Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Thomas Gleixner <tglx@linutronix.de> - 2016-10-08 16:20 +0200
          Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-08 18:50 +0200
        Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-08 16:20 +0200
    Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-18 14:50 +0200
      Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-18 15:00 +0200
      Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Chris Wilson <chris@chris-wilson.co.uk> - 2016-10-18 15:00 +0200
  [PATCH -v4 3/8] locking/mutex: Kill arch specific code Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:40 +0200
  Re: [PATCH -v4 0/8] locking/mutex: Rewrite basic mutex Linus Torvalds <torvalds@linux-foundation.org> - 2016-10-07 17:50 +0200
  Re: [PATCH -v4 0/8] locking/mutex: Rewrite basic mutex Jason Low <jason.low2@hpe.com> - 2016-10-11 21:10 +0200

csiph-web